CCI Concrete Testing & Evaluation 2 — Questions and Answers
Question 1: According to ASTM C138, what property of fresh concrete is determined by the unit weight test?
- Workability
- Air content (by calculation) (Correct answer)
- Compressive strength
- Water-cement ratio
Correct answer: Air content (by calculation)
ASTM C138 measures the unit weight (density) of fresh concrete, which can be used to calculate air content when the theoretical density is known.
Question 2: A concrete mix design specifies a w/cm ratio of 0.45. If the cementitious content is 600 lb/cy, what is the maximum water content allowed per cubic yard?
- 240 lb
- 270 lb (Correct answer)
- 300 lb
- 330 lb
Correct answer: 270 lb
Maximum water = w/cm ratio × cementitious content = 0.45 × 600 lb = 270 lb per cubic yard.
Question 3: Which ASTM standard governs the making and curing of concrete test specimens in the field?
- ASTM C31 (Correct answer)
- ASTM C39
- ASTM C78
- ASTM C143
Correct answer: ASTM C31
ASTM C31 covers the standard practice for making and curing concrete test specimens in the field.
Question 4: During a slump test, a concrete sample collapses to one side rather than slumping evenly. This is called a _____ slump and indicates what condition?
- True slump — acceptable workability
- Shear slump — possible segregation or poor cohesion (Correct answer)
- Collapse slump — excessive water content
- Zero slump — stiff mix
Correct answer: Shear slump — possible segregation or poor cohesion
A shear slump where the top slides off to one side indicates poor cohesion and possible segregation, making the test invalid and requiring a retest.
Question 5: At what age is concrete typically first tested for compressive strength in standard field practice?
- 3 days
- 7 days
- 28 days (Correct answer)
- 56 days
Correct answer: 28 days
The standard acceptance age for compressive strength is 28 days, as concrete reaches approximately 99% of its design strength by that time under normal curing.
Question 6: What is the minimum number of standard 4×8 inch cylinders that should be cast from a single composite sample to constitute one strength test per ACI 318?
- 1 cylinder
- 2 cylinders (Correct answer)
- 3 cylinders
- 4 cylinders
Correct answer: 2 cylinders
ACI 318 requires a strength test to consist of the average of at least two cylinders cast from the same composite sample and tested at the same age.
Question 7: When measuring air content of fresh concrete using the pressure method (ASTM C231), which type of aggregate requires a correction factor?
- Rounded gravel
- Crushed limestone
- Lightweight aggregate (Correct answer)
- River sand
Correct answer: Lightweight aggregate
Lightweight aggregate is porous and absorbs air pressure, requiring an aggregate correction factor when using the pressure meter method per ASTM C231.
CCI Concrete Testing & Evaluation 2
According to ASTM C138, what property of fresh concrete is determined by the unit weight test?